Metal speakers. How are they?

I notice that aluminum and magnesium cones have become very common, including in some high quality systems. What's the story on these things? I am considering purchase of a speaker that has Seas 7-inch magnesium woofers, in MTM configuration.

These speakers will replace Dynaudio Gemini MTM speakers whose little 5" woofers are great, but just won't keep up with my three MG1.6 + SW front speakers. I have contemplated rear subwoofers, but there just is not space. The Dynaudios were used for many years as the high end of a biamped system, and they were fine for that purpose.